1. Field of the Invention
The present invention relates to a method and an apparatus for broadcast receiving of a contract-based scramble type broadcast system and, in particular, such a method and an apparatus capable of contract management.
2. Description of the Background Art
A contract-based scramble type broadcasting utilizing a satellite station has been in the course of development. In such a broadcasting system, part of broadcast signals concerning the actual programs are scrambled but also accompanied by data required for de-scrambling the broadcast signals, so that only those who hold a valid contract can receive the broadcast by de-scrambling the broadcast signals with these required data.
The broadcast signal for such a broadcasting system is shown in FIG. 1(A), which comprises a video signal 10 and a audio signal 20. This audio signal 20 further comprises a synchronization code 21, an audio information 22, a related information packet 28, and an error correction code 24, as shown in FIG. 1(B). Such a broadcast signal is converted into codes by an encoder, transmitted from a broadcast station through the satellite station, and received by a receiver with a decoder for re-converting the broadcast signal from the codes.
The related information packet 28 contains related information, of which those concerning contracts are referred to as contract information whereas those concerning program and the data for de-scrambling the broadcast signal are referred to as a program information. These contract information and program information in the related information packet 23 are cryptogrammed for protection.
The program information 30 shown in FIG. 1(C) contains information on date of broadcasting 31, on program 32 including such information as a price of the program, a program number, and a category of the program, and the data for de-scrambling 33. This program information 30 will be transmitted from the broadcast station in every second, for example.
The contract information 40 shown in FIG. 1(D) contains information on ID of the decoder 41, on contract 42 including such information as a validation date and contracted programs, a program key Kw for deciphering the cryptogrammed program information 43, and on pre-paid fee 44. This contract information 40 will be transmitted from the broadcast station once every month, for example.
Conventionally, the decoder possesses a contract key Km for deciphering the cryptogrammed contract information 40, so that when the contract information 40 is received the cryptogrammed contract information 40 is deciphered by using this contract key Km, and the deciphered contract information is stored in a memory which is backed-up by a battery. A memory space 50 of such a memory is shown in FIG. 2, where there are program information storage 51, and a contract information storage 52 containing a room 53 for the pre-paid fee, a room 54 for the program key Kw, and a room 55 for contract information. As indicated in FIG. 2, the contract information storage 52 is battery backed-up so that the contract information 40 will not be lost accidentally.
Then, when the program information 30 is received a decision is made as to whether to allow the program to be received and consequently to de-scramble the actual program or not, in accordance with the stored contract information 40.
A flow chart for such a process of contract management is shown in FIG. 5, which is to be carried out by a microcomputer installed in the decoder.
First, at the step 60, the program information 30 and the contract information 40 in the received related packet information 23 are distinguished from each other. For the contract information 40, the step 62 will be taken next at which the cryptogrammed contract information 40 is deciphered by the contract key Km, and then at the step 63 the deciphered contract information 40 is stored into the contract information storage 52. On the other hand, for the program information 30, the step 64 will be taken next at which the cryptogrammed program information 30 is deciphered by the program key Kw. The deciphered program information 30 is stored into the program information storage 51 at the step 65, and then at the step 66 terms of the contract given by the contract information 40 in the contract information storage 52 are checked against prescribed conditions for allowing reception of this program. Whether all the conditions for the reception of the program are satisfied by this contract, in other words, whether this contract is valid for receiving this program, is determined at the step 67. If all the conditions are satisfied, the scrambled program is de-scrambled at the step 68 so that the program can be received, whereas otherwise the process terminates so that the program remains scrambled.
Such a conventional system of contract-based scramble type broadcasting has a phloem that because the contract information 40 is stored after being deciphered from the cryptogrammed form, the stored contract information 40 may be read out and altered rather easily, so that illegitimate receptions cannot be prevented very effectively.